The undetected propagation of a fatigue crack constitutes a significant cause of aircraft and other structural failures. To raise the structural failure load to a relatively high level, the manufacturer can divide the structure into many small elements, which significantly increases the ability of a structure to tolerate an element failure. This paper presents a procedure for calculating the probability that the structure has not failed, as the function of the crack propagation history for an undetected fatigue crack. The form of the procedure is so simple that computations with a desk calculator can yield reasonably accurate results. Moreover, the necessary input data are often readily available. By adopting such a procedure, aircraft manufacturers and operators can better identify those elements that pose the greatest threat to structural integrity.